A serious problem confronting electrical utility companies today is the inability to handle the excessive demand placed on an electrical power distribution system during peak demand hours. Periods of excessive demand occur, for example, during very hot summer days when simultaneous usage of air conditioning units is widespread. The extreme demand placed on a power distribution system during such peak demand periods can lead to service interruptions, such as "brown-outs." In an effort to prevent or minimize service interruptions during peak demand periods, utility companies are beginning to employ remotely controllable load management terminals at selected customer locations.
A typical load management terminal has a relay connected to the power line in series with the load. The types of loads most often targeted in a load management system are large appliances such as water heaters and air conditioning units. During peak demand periods, a utility can transmit a command to a load management terminal causing that terminal to open the relay and prevent current from flowing to the load, thereby "shedding" the load from the power line. Some load management terminals are adapted to receive commands directly over the power line, for example, on a high frequency carrier. Other load management terminals employ radio receivers for receiving remote commands.
Participation by a consumer in a utility's load management program is often voluntary; that is, the consumer agrees to let the utility install a load management terminal in return for some form of credit or rebate on the consumer's electric bill. Once a load management terminal has been installed for a particular load, the utility can begin to remotely shed that load at various times during a peak demand period. Utilities try to perform load shedding in such a way as to minimize the discomfort and inconvenience to the customer. However, some discomfort and inconvenience is inevitable.
Unfortunately, some customers will accept a load management terminal in order to obtain financial benefit from the utility and then will tamper with the load management terminal in an attempt to disable it. Consequently, utilities have realized the need to detect such tampering efforts easily without significantly adding to the cost of the terminal. As can be expected, there are many ways to tamper with a load management terminal, and therefore, tamper detection methods and apparatus depend on the particular form of tampering that the utility is trying to detect.
Virtually all load management terminals have some form of outer housing that surrounds the load management relay and other internal components. Tampering by adjusting or circumventing the internal components, therefore, will require opening the terminal housing. Consequently, most load management terminals provide some sort of tamper indication when the outer housing has been opened or breached in some manner. For example, Rudden et. al., U.S. Pat. No. 4,977,515, describes the use of a sensor to detect opening of the housing. Stanbury et. al., U.S. Pat. No. 4,850,010, also describes detecting the opening of a terminal's housing. Additionally, many current load management terminals are microprocessor based and contain some amount of electronic memory. Subjecting such a load management terminal to a severe magnetic field can disrupt the electronic memory circuits and otherwise interfere with the ability of the load management terminal to shed its load. Aforementioned U.S. Pat. No. 4,997,515 discloses a means for detecting this form of tampering. Essentially, a Gauss detector is used to measure magnetic fields present in the terminal and to signal a microprocessor whenever a magnetic field strong enough to disrupt the terminal is detected.
Another form of tampering with load management terminals involves the connection of a by-pass link to the power line in parallel with the load control relay of the load management terminal. Such by-pass links effectively take the load control relay out of the circuit because current is shunted around the relay. Thus, when the normally closed contacts of the load control relay are opened in response to a load shed command, current will still be supplied to the load through the by-pass link. This last form of tampering is not adequately addressed in the prior art of load management. In the somewhat related area of electrical energy consumption meters, however, the problem of meter tampering using a by-pass link around the meter has been addressed. However, the solutions have been complex and expensive, and therefore, unsatisfactory for use in load management terminals.
For example, Fielden, U.S. Pat. No. 4,331,915, describes one method for detecting the presence of a by-pass link across the live connection through an electrical watt-hour meter. According to the method of Fielden, a voltage transformer in the meter is used to induce a small voltage in the live connection in opposition to the normal supply voltage. In the absence of tampering, the induced voltage has a negligible effect. When a by-pass link is connected, however, the induced voltage produces a circulating current in the loop formed by the live connection and the by-pass link. This induced current will be in phase opposition to the normal supply voltage. A phase comparator is used to compare the phase of the induced current with the phase of the supply voltage. A detected phase opposition indicates tampering. While this solution may be satisfactory for electrical watt-hour meters, it is not satisfactory for use in load management terminals. Fielden's method requires too much additional hardware, and therefore, is relatively expensive. Cost is more of a concern with load management terminals than with electrical watt-hour meters because, in addition to the cost of the terminal, utilities must also provide a credit to consumers who agree to have such terminals installed. Accordingly, the tamper detection method of Fielden does not provide a satisfactory solution for load management terminals.
Hurley, U.S. Pat. No. 4,532,471, describes another tamper detection method for electrical meters. Again, however, this solution is unsatisfactory for use with load management terminals due to its complexity and resulting cost. Hurley employs a current transformer and associated electronics to sense a change in impedance that results from the connection of a by-pass link around the meter. The current transformer is coupled to the power line conductor within the meter such that the power line forms a primary winding for the transformer and defines a primary circuit. The multi-turn secondary winding on the current transformer defines a secondary circuit. A by-pass link connected across the meter will cause a change in the impedance reflected from the primary circuit into the secondary circuit. A measuring device is connected to the current transformer for detecting such changes. Although this method achieves tamper detection, it too is not cost efficient for use in load management terminals due to its complexity.
A need arises, therefore, for methods and apparatus for detecting tampering with load management terminals that does not significantly add to the cost of manufacturing such terminals. In particular, cost efficient methods and apparatus are needed for detecting the connection of a by-pass link to the power line in parallel with the load control relay of the load management terminal in an effort to "short-circuit" around the relay. The present invention satisfies these needs.